Ultrasound Imaging Flashcards
Describe the physics behind Ultrasound acquisiton for clinical imaging
- Wavelength and freq
- diagnostic ultrasound imaging: 2MHz to 15Mhz - Propagation of sound
- sound = mechanical wave (uses pressure to move the actual particles longitudinally)
- avg speed in soft tissue = 1540 m/s
wavelength = speed / freq
Describe the cause of speckle noise in ultrasound imaging and how to reduce it
Constructive and destructive interference of scatted sound by diffuse reflectors causes speck noise
- increased intensity –> white
- decreased intensity –> black
How to reduce it:
- ultrasound transducers use frequency and spatial compounding
(digitally steer the US beam to take images at diff angles and combine them into 1)
describe impact / application of reflection and refraction in ultrasound imaging
Acoustic impedence = resistance against passing through of sound waves
Z = density * velocity
Reflection
- results from change in acoustic impedance
-(increased acoustic impedence difference = higher reflection)
- Specular reflectors
- caused when US waves hit smooth and large surfaces at an angle –> less resolution in that region
- ratio of reflected energy:
R = (Zf - Zi)^2 / (Zf + Zi)^2 - Diffuse reflectors
- US sound wave is scattered at many angles
Refraction
- results from change in propagation velocity (causes US wave direction to be different as it passes through different mediums)
- sin A / sin B = Ca / Cb
Remedy for refraction
- image from different angles
(one of those angles will experience the least refraction)
ex. Ultrasound of uterus requires precise position in between the rectus abdominis muscle, otherwise 2 sacs (instead of 1) will show up
Identify building blocks of an ultrasound machine
- Beamformer
- digital steering / focusing
- beam summation - Pulsers
- sends voltage to US transducer
3.Receiver
- time gain
- log compression
- rectification
- rejection
- Scan converter
- memory
- digital interface
- post processing
- storage
Describe sound propagation properties for different soft tissues and how it is used in ultrasounds
propagation of sound –> depends on density and stiffness/elasticity of material
(bones > muscle > blood > kidney > liver > avg tissue > water > fat > air)
Application = distance measurement (echo-ranging principle)
- rapid repetition gives 2D map of reflecting interfaces
- assumes straight path to and from reflecting interface and 1540 m/s
- eq: d = (1/2) * c * dTime/dt
What is propagation velocity artifact?
when US passes thorugh a fatty region, it will pass through slower
–> results in longer time to get reflected back to the US detector –> calculated distance is farther than in actuality
Describe attenuation and intensity of ultrasounds
Attenuation –> how much the amplitude of a US signal gets reduced as it passes through tissue
- caused by absorption, scattering, and reflection
- depends on insonation freq and medium properties
air > bone > transverse m.> parallel m.> kidney > liver> avg tissue> fat > blood > water
Intensity –> measure of spatial distribution of acoustic energy over time
I = Power (W) / Area (cm^2)
What are the key functions of ultrasound scanner (4)?
- transmitter / pulser energizes transducer
- receiver and processor detects, amplifies, and manipulates the echo signals
- display for analysis and interpretation
- record and/or store ultrasound image
Explain how Ultrasound transmitters work
- Pulsed US
Clinical applications use pulsed ultrasound (sends burst and listens for back scatter) - High Voltages
Transducer energized by precisely-timed, high-amplitude voltage ( around 1000 V)
- max voltage under federal regulation (potential risk)
- scanners have control of attenuation of output voltage
- ideal: use lowest levels sufficient for each diagnostic problem - Repetition
Transmitter controls Pulse reptiiton frequency (RPF)
- measure of time interval between pulses
- used to determine US depth
- ranges from 1 to 10 kHz
A clinician is operating an ultrasound machine at PRF of 5kHz. How far is the depth at which unambiguous data can be obtained?
Assume sound travels at 1540 m/s in tissue
Unambiguous = meaningful data
- needs time between pulses to listen to back scatter
Calculation:
depth = (1/2) * speed * period
(period = time between pulses = 1 / frequency)
D = (1/2) * (1540) * (1 / 5)
Describe how ultrasound probe acts as a transducer and receiver (both on the same end of the probe)
converts electric energy to mechanical energy and vice versa
Materials:
Piezoelectric materials
- changes shape in response to electric field
- generates electricity (must be amplified) when compressed
Transducer (generating mechanical waves)
- changing polarity changes thickness (expand / contract) –> creates band of frequencies
Receiver (generating electric voltage)
- positive small potential: compressions
- negative small potential: rarefaction
Describe the band of frequencies generated by the transducer
preferential freq of transducer determined by:
- propagation speed
- thickness of transducer material
Shorter pulses of ultrasound –> larger bandwidth of frequencies
broad-bandwidth
- used tocover tissues w/ different freq spectrum bandwidth
low freq
- high penetration
- low resolution
Describe the pulse-wave feature of diagnostic ultrasounds and how we can use time between pulses for diagnostics
Pulse-wave:
transducers vibrate for a short time after stimulation –> pulse will be several cycles long
Pulse duration
-time version of spatial pulse length
- usually 1 - 2 us
Pulse repetition period (PRP)
- how often we spend pulses
- typically 250 to 500 us
Each pulse will echo and be received at different times within the PRP
- longer time = greater depth
Explain how spatial pulse length is related to axial (depth) resolution
Spatial Pulse Length: (SPL)
- distance that pulse occupies in space
- product of wavelength and #cycles per pulse
Axial resolution
- ability to distinguish between closely spaced objects in direction of US beam
- equal to 1/2 SPL (allows the first wave-pulse to echo back without overlap of pulses)
How do you acheive shorter SPL
- damping of transuder elements
- can get as short as 2 to 3 cycles
- results in higher freq of operation(shorter wavelength)
less cyles = shorter SPL therefore higher resolution
Compare and contrast Near vs far field operations of US transducers
Concept:
- 3D acoustic waves tend to diverge
- causes near and far fields
Near field (smaller distances from transducer)
- parallel waves
- increased resolution
Far field
- after focal zone
- waves start to diverge
- backscatter is inaccurate (decreased resolution)
How do the materials of a transducers help address divergence of waves (far fields)
Modern multielement transducers:
1. precise timing of the firing of elements
- effect: corrects divergence and improves focus at selected depths
- matching impedence between skin and transducers
- effect: reduces backscatter / reflection
Describe the 3 different types of beam steering
A) linear array
- used for vascular and obsteric regions
- flat transducer and grid-like US waves
B) curved array
- used for variety of applications
- curved transducer and fanned out US waves
C) Phased array
- used for neonatal head and intercostal scanning
- flat transducer and fanned out US waves (requires digital beam manipulation)
Compare and contrast the different ultrasound image displays (2)
- M- mode (motion mode)
- shows changes of echo amplitude and position w/ time
- sends only 1 line of US waves - B- mode (brightness mode)
- multiple scan lines creates a 2D image
- image created 15 to 60 times / second for real time imaging
- strength of backscattered signal encodes for brightness level
Describe ultrasound elastography and compare the different types (2)
Concept:
- palpation detects change in tissue stiffness
- stiffness proves relative / quantitative measure of young modulus
Strain elastography
- manually apply pressure
- relative measure of Young modulus
Shear wave elastography
- sends high freq wave to tissue and observes how resulting shear wave travels in tissue
- provides quantitative measure of young modulus
Describe the concept of doppler sonography
Concept:
- shift in freq is caused by motion of target relative to incident beam
dF = Fr - Ft = 2(Ft)(v)cos(theta / c)
Fr –> US freq obs by receiver
Ft –> US freq sent by transducer
recommended range of theta
–> 0 to 60
(90 results in no change in freq)
* higher amp signal if US waves travel in same dir as blood flow
compare doppler sonography results between stationary and moving RBC
- stationary
(Fr - Ft) = 0 - moving towards US probe
(Fr - Ft) > 0 - moving away from US probe
(Fr - Ft) < 0
Why are pulse wave US used for doppler donography but not continuous wave US
continuous waves fail at doppler sonography
- unable to distinguish signal from vessels at different lengths
(wavelength «< 1/2)
Describe Color Doppler imaging
- Initialize brightness values
Amplitude data from stationary targets provides bassis for B-mode image - Detect
signal phase provides info abt:
- presence of motion
- direction of motion
- changes in freq –> velocity of target
